Panasonic Corp. will begin volume production of Li-ion rechargeable batteries using a silicon (Si) alloy anode in fiscal 2012. Intended for use in notebook PCs, the battery cells will measure 18mm in diameter x 65mm long (18650 size), and offer a high 4.0Ah capacity. The largest capacity of any cell of that size available now is only 3.1Ah, so that the Panasonic design represents a significant improvement of close to 30%.
Si alloy has a theoretical capacity at least ten times greater than the graphite now most commonly used in anodes. Most battery manufacturers are now developing it as a next-generation material, but so far Panasonic is the only company to announce specifications of product.
Ensuring a 500-Cycle Service Life
Researchers are having problems developing a practical Si-alloy anode because of the large volumetric change during charge and discharge. Li-ion movement during charge and discharge can cause a volumetric change of about 400% in Si-alloy materials. This makes electrode structural collapse more likely, and slashes the charge/discharge cycle life.
Panasonic improved fabrication techniques and made other modifications to reduce the characteristic deterioration caused by this volumetric change. The concrete improvements have not been disclosed, but the firm claims that even after 500 charge/discharge cycles at least 80% capacity is retained.
The initial target application will be notebook PCs, but a source at the firm comments "the ultimate target is vehicles." For vehicular applications, weight will be a key issue. The current design weighs about 54g per cell, which is about 10g more than the company's prior model (NCR18650A). The weight energy density is about 252Wh/kg, or about the same as existing graphite designs, indicating that the full potential of Si has yet to be attained.
Even so, it is clear that Si alloy is a highly promising material for the future. Already Hitachi Maxell, Ltd. has announced plans to begin volume production by the end of fiscal 2009. The commercial launch announced by Panasonic will only further accelerate the Si-alloy development competition.
Source: Nikkei Business Publications
